Genetic variation, variation in alleles of genes, occurs both within and among populations. Genetic variation is important because it provides the genetic material for natural selection.There are two types of variation

1) Continuous

2) Discontinuous

ContinuousVariation within a population in which a graded series of intermediate phenotypes falls between the extremes.

As a matter of fact, if you consider a large enough sample from a population, perhaps plotting frequency as a histogram or as a frequency polygon, you will find that most of the values are close to the average (mean), and extreme values are actually rather rare.

Height is an example of a continuously variable characteristic, as long as you consider a consistent sample, for example a large number of people of a particular age and sex.As you will find out later, it is usually difficult to give a straightforward explanation of the genetic basis for these continuously variable characteristics because they result from a combination of genetic factors as well as environmental influences. DiscontinuousCharacteristics showing discontinuous variation fall into a few very distinct classes. The ability to roll the tongue, and blood groups, are examples of discontinuous variation. These characteristics can be explained much more easily by simple rules of genetics and are less likely to be affected by other factors.

Factors affecting variation

These can either be:

1) Environmental

2) Genetic

Environmental Factors

The environment in which a plant or an animal was reared or grown can affect the expression of certain gene and as a result give rise to variation. Example plants of the same species grown in light and dark will look differently

Genetic Factors

Genetic variation can be produced by the recombination of chromosomes that occurs during sexual reproduction, called independent assortment.Independent assortment occurs in eukaryotic organisms during meiotic metaphase I, and produces a gamete with a mixture of the organism's chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent chromosome along the metaphase plate with respect to the other bivalent chromosomesCrossing over during meiosis can result in the production of new alleles or new combinations of alleles. Furthermore, random fertilization also contributes to variation.

Mutations are the ultimate source of genetic variation because they alter the order of bases in the nucleotides of DNA. Mutations are likely to be rare and most mutations are neutral or deleterious, but in some instances the new alleles can be favored by natural selection.

There are different types of mutation:

1) Gene mutation

2) Chromosomal mutation

Gene MutationThree types, this type of mutation result in a change in the DNA sequence

Substitution

A substitution is a mutation that exchanges one base for another (i.e., a change in a single "chemical letter" such as switching an A to a G). Such a substitution could:

change a codon to one that encodes a different amino acid and cause a small change in the protein produced. For example, sickle cell anemia is caused by a substitution in the beta-hemoglobin gene, which alters a single amino acid in the protein produced.

change a codon to one that encodes the same amino acid and causes no change in the protein produced. These are called silent mutations.

change an amino-acid-coding codon to a single "stop" codon and cause an incomplete protein. This can have serious effects since the incomplete protein probably won't function.

Insertion

Insertions are mutations in which extra base pairs are inserted into a new place in the DNA.

Deletion

Deletions are mutations in which a section of DNA is lost, or deleted.

Chromosomal Mutation

Chromosome Mutations result in change in the number of chromosomes incorporated into sex cells. A child produced as a result may have, for instance, an extra chromosome, or an extra part of a chromosome attached to the normal set.

Down's syndrome is caused by having 47 chromosomes instead of the normal 46 per cell.